Gas chromatography is an advantageous method for analyzing minute quantities of complex mixtures from biological and chemical sources. Gas chromatography may be used to determine the constituents in the mixture from which the sample is taken. Gas chromatography typically involves volatizing a sample to be analyzed and moving the sample in a stream of inert carrier gas.
The sample and carrier gas are delivered to a packing bed or column in the gas chromatograph. The different constituents in the sample move through the column at different rates. As a result, they are separated and appear one after the other at the output end of the column. At the outlet end the separated materials in the sample are identified through their properties of thermoconductivity, density differences or by ionization detectors, depending on the type of gas chromatograph.
Headspace sampling is a technique that involves testing a gaseous sample generated in a closed container above a solid or liquid substance to be analyzed. Sampling gaseous material avoids the introduction of non-volatile or solid particles into the inlet of the gas chromatograph, which is not desirable.
Headspace sampling involves establishing an equilibrium in a sealed vial between the substance to be analyzed and the volume above the substance. Equilibrium is established by heating the sample in the vial. A gaseous sample is then taken from the headspace and delivered to the analytical instrument.
This technique has advantages because it assures that only gaseous material enters the gas chromatograph or other instrument. Further, because the sample is vapor, the amount of the sample may be much larger than a liquid sample. This increases the sensitivity of the analysis. These advantages of headspace sampling make it a useful technique in the analysis of polymers, latexes, paints, foods, biological materials, environmental samples, pharmaceuticals, fragrances and other substances.
Headspace sampling has certain limitations however. First, the volatility of a particular substance at a given temperature varies depending on the matrix of other materials it is mixed with. For example, the same concentration of benzene in water and in gasoline will have very different concentrations of benzene in the headspace at the same temperature. Therefore, in the prior art, it has been necessary to prepare calibration samples to simulate the matrix of materials in the substance being analyzed. However, this is impossible in cases of a solid sample or a substance that is completely unknown.
A further drawback of current headspace sampling techniques is that it takes many samples a long time to reach equilibrium between the condensed and vapor phases. Achieving equilibrium with current equipment often requires hours of heating. This is particularly true for viscous or solid substances. Some have tried agitation of the sample and vial in an attempt to shorten equilibrium times, but this has not proven effective and sometimes causes contamination problems.
Several approaches to headspace sampling have been used in the prior art. Each of these approaches suffers the drawback that removal of the gaseous sample from the headspace disturbs the equilibrium. Such disturbance of the equilibrium can cause poor repeatability. A further problem common to prior art techniques is that a disturbance in equilibrium may vary from sample to sample depending on the technique used and the properties of the materials being analyzed.
One approach to headspace sampling used in the prior art is syringe sampling. This technique involves removing a sample of gaseous material from the headspace over a substance to be analyzed, using a syringe. The disturbance of the equilibrium in the headspace using this technique depends on the amount of headspace in the sample vial compared to the volume of the sample. It also depends on the speed of removal of the sample, as removal of the material from the headspace will tend to cause more of the substance to enter the vapor phase to establish a new equilibrium.
A further problem with the syringe technique is that when the syringe is removed from the vial, some of the vapors will expand and escape to atmosphere prior to injection of the sample into the inlet port which carries the sample into the column of the gas chromatograph. Another drawback is that material in the sample may begin to condense due to cooling before the sample enters the gas chromatograph. As a result, not all the material in the sample may be delivered, which causes poor repeatability.
Another headspace sampling technique used in the prior art is explained with reference to FIG. 1. This approach is called fixed volume injection. The fixed volume system 10 has a sample vial 12 with a substance 14 therein. The vial is generally a cylindrical vial which is held in the upright position during sampling. This is standard with all prior art headspace sampling techniques.
A sampling needle 16 is positioned in the headspace in the interior volume of the vial above the substance. The needle is in fluid communication with a first port 20 of a six port valve 22. Six port valve 22 alternatively places first adjacent ports, shown connected by the solid lines in the drawing, in fluid communication when the valve is in a first condition. In a second condition of the valve, the alternative adjacent ports shown connected by the dashed lines are connected.
The second and fifth ports of the valve 24 and 26, respectively, are connected by a sample loop 27. The third port 28 is connected to the inlet of the gas chromatograph. The fourth port 30 is connected to a source of inert carrier gas such as helium. The sixth port 32 is connected to a valve 34 that is alternatively connected to a source of inert gas or to atmosphere.
In operation, with valve 22 in the condition shown in the drawing, the inert gas pressure of port 32 is applied to headspace 18 through port 20. In this condition of valve 22, the carrier gas passes through the sample loop 27 to the gas chromatograph inlet through ports 30, 26, 24 and 28.
The conditions of valves 22 and 34 are then changed. In these alternative conditions, the headspace sample is directed into the sample loop through connection of ports 20 and 24 of the valve. The sample loop 27 also vents to atmosphere due to the connection of ports 26 and 32 and the opening of valve 34. As a result, sample loop 27 is filled with gaseous material from the headspace of the vial.
The return of valve 22 to the first condition causes the carrier gas to wash the sample material in the sample loop into the inlet of the gas chromatograph. This is accomplished because the carrier gas is connected to port 26 and pushes the material in the sample loop through ports 24 and 28.
The operation of the system 10 includes several timed functions including the time of heating the sample to attempt to achieve equilibrium, pressurization time for the headspace, time of venting the headspace vapors through the sample loop and the time of washing the sample loop with the carrier gas. All of these timed event functions impact the results produced by the gas chromatograph. Particularly problematic is that venting the sample to atmosphere disturbs the headspace equilibrium. Also, it is a common problem that the headspace vapor passing through the lines and valves of the system, will begin to condense, which further adversely affects repeatability and cross contamination.
An alternative type of prior art headspace sampling system 36 is shown in FIG. 2. In this system, a vial 38 contains a substance 40 to be analyzed. The headspace 42 is pressurized with carrier gas through a sampling needle 44. At the same time as the headspace is pressurized, the column of the connected gas chromatograph 46 is pressurized to the same pressure. Carrier gas flow to the vial and to the gas chromatograph is then shut off. Thereafter the headspace 42 of the vial is the source of gas delivered to the gas chromatograph.
This type of sampling is suitable for use with gas chromatographs that have high column pressures. However, many gas chromatographs have low column pressures which make this prior art approach unsuitable. This is because at low pressures the amount of sample delivered into the column of the gas chromatograph is too small to produce accurate results.
In conclusion, prior art sampling devices have inherent problems due to the effects of condensation or loss of the constituents of the sample before entry into the gas chromatograph. Prior art systems also have the drawback that long heating times are required to insure that the headspace reaches equilibrium with the substance to be analyzed.
Thus there exists a need for a headspace autosampling apparatus that provides greater sensitivity and repeatability, reduces cycle times and is readily adaptable for use with a variety of substances and types of analytical instruments.